[DownloadStep] Button to download file list if non available

[bruderstein]

When opening a plugin to edit with a download step, the list of available files is not automatically downloaded. However, in order to keep track of which file hashes must be added to the plugin, we need that file list if any alterations are to be made to following copy steps. There should be a button that allows downloading the file list if it's not available. It's currently downloaded when the URL is changed (debounced so only after the user stops typing), so it's just a matter of triggering the same thing on the button click